Imaging systems for converting infrared radiation, invisible to the unaided human eye, to visible radiation have found wide applications. For example, infrared sensors have been utilized as intrusion detectors wherein it is desired to identify an intruder in a secure area without requiring active illumination; for medical diagnosis such as in mammography; and other applications as described, for example, in U.S. Pat. No. 2,895,049.
Prior art infrared imaging systems, such as that disclosed in the aforementioned patent and U.S. Pat. No. 3,509,345, generally incorporate an infrared detector which is caused to optically scan over the field of view, producing electrical signals in accordance with the detected infrared radiation. The electrical signals from the detector are processed and applied to a light generating means which generates a visible image corresponding to the detected infrared radiation. The generated visible image may be recorded (or displayed) by scanning the visible image in synchronism with the detected infrared image and focusing the visible image on the light sensitive surface of a film, for example.
The infrared imaging device described in the aforementioned patents utilize a single, double-sided two axis scanning mirror to both scan the infrared field of view and to record the corresponding visible image on a recording medium. Although scanning of the field of view and the recording medium are accomplished simultaneously, eliminating synchronization problems, the use of a single double-sided scanning mirror limits both the speed of scan and the field of view which can be covered by the scanner. In particular, if full-field coverage is required by the x-direction and y-direction scanning mirror, the single mirror must be gimballed in both the x-direction and the y-direction, the drive mechanism for the inner gimbal automatically becomming part of the load which must be driven by the outer gimbal drive. The scanner drive mechanisms required to provide full field coverage are either galvanometers or dc torque motors and, in either case, the mass of these devices will add to the inner gimbal a load many times the mirror mass. Thus, this combined driver/mirror mass on the inner gimbal will considerably increase the inertia which must be driven by the outer gimbal drive mechanism (outside gimbal). This increased mass will greatly reduce the attainable scan rate on the outside gimbal, in addition to the increased power which will be required to drive it. Further, many of the prior art systems which utilize a single double-sided scanning mirror (with detectors and light emitters) use principally a linear array of equally spaced infrared detectors which are placed over the entire image height of the system. With the detector array placed over the full height of the field of view (FOV), the y-direction motion requirement is reduced to the very small motion required to scan the distance between detectors (interlace) as opposed to scanning the entire y-direction, FOV. Although the required y-direction motion is less, a possible disadvantage with the aforementioned detector arrangement makes its used less attractive. In particular, when a channel (portion of system which electrically processes the output signal from an infrared detector element) becomes disabled or the channel gain varies, light or dark raster line or lines will appear in the output imagery, reducing the resolution and information content thereof.